1. Field of the Invention.
The present invention relates to active pixel sensor cells and, more particularly, to an active pixel sensor cell with a balanced blue response and reduced noise.
2. Description of the Related Art.
Charge-coupled devices (CCDs) have been the mainstay of conventional imaging circuits for converting a pixel of light energy into an electrical signal that represents the intensity of the light energy. In general, CCDs utilize a photogate to convert the light energy into an electrical charge, and a series of electrodes to transfer the charge collected at the photogate to an output sense node.
Although CCDs have many strengths, which include a high sensitivity and fill-factor, CCDs also suffer from a number of weaknesses. Most notable among these weaknesses, which include limited readout rates and dynamic range limitations, is the difficulty in integrating CCDs with CMOS-based microprocessors.
To overcome the limitations of CCD-based imaging circuits, more recent imaging circuits use active pixel sensor cells to convert a pixel of light energy into an electrical signal. With active pixel sensor cells, a conventional photogate is typically combined with a number of active transistors which, in addition to forming an electrical signal, provide amplification, readout control, and reset control.
FIG. 1 shows a cross-sectional and schematic view of a conventional CMOS active pixel sensor cell 10. As shown in FIG. 1, cell 10, which is formed in a p-type substrate 12, includes a pair of spaced-apart n+ regions 14 and 16 formed in substrate 12, a polysilicon (poly) photogate 18 formed over and insulated from substrate 12 adjacent to n+ region 14, and a poly transfer gate 20 formed over and insulated from substrate 12 between n+ regions 14 and 16. In addition, photogate 18 is formed to receive a photogate signal PG, while transfer gate 20 is formed to receive a transfer signal TX.
As further shown in FIG. 1, cell 10 also includes a reset transistor 24 having a source connected to n+ region 16 and a gate connected to receive a reset signal RST, a source-follower transistor 26 having a gate connected to n+ region 16, and a row-select transistor 28 having a drain connected to the source of source-follower transistor 26 and a gate connected to receive a row-select signal RS.
The operation of cell 10 begins with an integration period during which light energy in the form of photons penetrates substrate 12 and forms a number of electron-hole pairs. Throughout the integration period, a positive voltage is applied to photogate 18 via photogate signal PG to attract the newly formed electrons to the surface of substrate 12 directly below photogate 18.
Following the integration period, the voltage on n+ region 16 is reset to an initial transfer voltage by pulsing the gate of reset transistor 24 with a positive voltage via reset signal RST. The initial transfer voltage placed on n+ region 16, in turn, defines an initial intermediate voltage on the source of source-follower transistor 26.
Immediately after the gate of reset transistor 24 has been pulsed, the gate of row-select transistor 28 is pulsed with a positive voltage via the row-select signal RS. The positive voltage on the gate of row-select transistor 28 causes the initial intermediate voltage on the source of source-follower transistor 26 to appear on the source of row-select transistor 28 as an initial integration voltage which, in turn, is read out and stored by an imaging system.
Following this, a positive voltage is applied to transfer gate 20 via the transfer signal TX to slightly invert the surface of substrate 12 under transfer gate 20. At the same time, a negative voltage is applied to photogate 18 via the photogate signal PG.
The negative voltage applied to photogate 18 in combination with the positive voltage on n+ region 16 causes the charge collected under photogate 18 to flow through the inverted surface region under transfer gate 20 via n+ region 14 to n+ region 16 where each incoming electron reduces the initial transfer voltage on n+ region 16 to a final transfer voltage.
The final transfer voltage on n+ region 16 is then read out via source-follower transistor 26 as a final integration voltage by again pulsing the gate of row-select transistor 28. As a result, a collected voltage which represents the total charge collected by photogate 18 can be determined by subtracting the final integration voltage from the initial integration voltage.
One of the advantages of cell 10 is that the collected voltage is relatively free from noise. There is essentially no noise associated with photogate 18, and the noise associated with n+ region 16 is minimized by quickly reading the first integration voltage, transferring the charge to n+ region 16, and then reading the second integration voltage.
One problem with cell 10, however, is that photogate 18, which is formed from polysilicon, tends to block 50% or more of the highly energetic blue photons. As a result, the reduced number of blue photons leads to an unbalanced blue response.
One technique for improving the problem of an unbalanced blue response is to utilize a conventional photodiode, which does not require a layer of polysilicon, in lieu of a photogate. FIG. 2 shows a cross-sectional and schematic diagram that illustrates a photodiode-based active pixel sensor cell 50.
As shown in FIG. 2, cell 50, which is formed in p-type substrate 52, includes an n-type region 54 formed in substrate 52, an n+ contact region 56 formed in substrate 52 adjacent to n-type region 54, and a p+ region 58 formed in n-type region 54. Although not required to form a photodiode, p+ region 58 is commonly used to passivate the Si/SiO.sub.2 interface.
As further shown in FIG. 2, cell 50 also includes a reset transistor 60 having a source connected to n+ region 56 and a gate connected to receive a reset signal RST, a source-follower transistor 62 having a gate connected to n+ region 56, and a row-select transistor 64 having a drain connected to the source of source-follower transistor 62 and a gate connected to receive a row-select signal RS.
Operation of active pixel sensor cell 50 is performed in three steps: a reset step, where cell 50 is reset from the previous integration cycle; an image integration step, where the light energy is collected and converted into an electrical signal; and a signal readout step, where the signal is read out.
During the reset step, the potential on n-type region 54 and n+ contact region 56 is raised to an initial transfer voltage by pulsing the gate of reset transistor 60 with a positive voltage via the reset signal. As above, the initial transfer voltage placed on n+ region 56 also defines an initial intermediate voltage on the source of source-follower transistor 62.
Immediately after the gate of reset transistor 60 has been pulsed, the gate of row-select transistor 64 is pulsed with a positive voltage via the row-select signal RS. The positive voltage on the gate of row-select transistor 64 causes the initial intermediate voltage on the source of source-follower transistor 62 to appear on the source of row-select transistor 64 as an initial integration voltage which, in turn, is read out and stored by an imaging system.
Following this, the integration step begins. During integration, light energy, in the form of photons, penetrates into p+ region 58, n-type region 54, and substrate 52, thereby creating a number of electron-hole pairs.
The photogenerated electrons formed in p+ region 58 and substrate 52 which diffuse into the junction regions are attracted to n-type region 54 and n+ contact region 56 under the influence of the junction electric fields, while the photogenerated electrons formed in n-type region 54 remain in n-type region 54 where each additional electron reduces the potential on n-type region 54 and n+ contact region 56.
Thus, at the end of the integration step, the potential on n-type region 54 and n+ contact region 56 will have been reduced to a final transfer voltage where the amount of the reduction represents the intensity of the received light energy.
Following the image integration period, the final transfer voltage on n+ region 56 is then read out via source-follower transistor 62 as a final integration voltage by again pulsing the gate of row-select transistor 64. As a result, a collected voltage which represents the total charge collected by cell 50 can be determined by subtracting the final integration voltage from the initial integration voltage.
One of the problems with active pixel sensor cell 50, however, is that the period conventionally used to integrate the image, i.e., the time between reading out the initial and final integration voltages, which is approximately 30 mS, introduces noise into the collected voltage.
Thus, there is a need for an active pixel sensor cell that provides a balanced blue response and at the same time minimizes the introduction of noise into the collection voltage.